As portability of electronic products becomes important, a lithium secondary battery has been widely used as a power of electronic devices having a small size, such as notebook PC, PDA, mobile phone, video camera, and the like, and due to properties of high capacity, high output, long-term life span, and the like, an application range thereof has been extended. However, the lithium secondary battery has a difficulty in being used for a long-term due to limitation of the existing electrode materials used in the lithium secondary battery. In order to overcome the above-described problem, various researches have been conducted, and in particular, development of a new cathode active material has been variously researched.
A representative material of the cathode active material of the lithium secondary battery is lithium cobalt oxide (LiCoO2), and as substitutes thereof, lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), or lithium nickel cobalt manganese oxide (LiNi1-x-yCoxMnyO2), each having a layered structure, lithium manganese oxide (LiMn2O4) having a spinel structure, lithium iron phosphate oxide (LiFePO4) having an olivine structure, and the like, have been actively researched.
Since a lithium cobalt oxide (LiCoO2)-based active material has problems such as high raw material cost, and low thermal stability of cobalt, it is difficult to secure cost, capacity, or stability required in medium- and large-sized electric vehicle market such as hybrid electric vehicle (HEV) and electric vehicle (EV).
In order to substitute the lithium cobalt oxide active material, a manganese-based spinel or an olivine structure using iron have come to the front. However, a cathode active material having a spinel structure using manganese has significantly deteriorated capacity due to elution of manganese at high temperature and has a capacity relatively smaller than that of LiCoO2. In addition, the olivine-based cathode active material has advantages such as raw material cost reduction and high thermal stability due to the use of iron; however, problems such as low driving voltage and low electric conductivity.
Patent document 1 (Korean Patent Laid-Open Publication No. KR 10-2010-0042145) discloses a cathode active material for a lithium secondary battery containing a lithium transition metal composite oxide, wherein the cathode active material contains manganese, and halogen element or sulfur to improve life span properties and charge and discharge properties of the lithium secondary battery; however, due to dispersion, structural and thermal stability are still deteriorated and electrochemical properties such as capacity deterioration, and the like, are poor.
Meanwhile, the cathode active material using nickel only is appropriate for a battery for high capacity and high voltage but has capacity deterioration occurred due to cation mixing and structural instability and weak thermal stability.
As a nickel-based cathode active material, a composition having LiNi1-xCoxO2 (x<0.2) is a representative example, and since cost of cobalt with respect to nickel is high, the cathode active material has a relatively high cost as compared to an LiNiO2 material not containing cobalt, but is easy to be prepared. Nonetheless, LiNi1-xCoxO2 (x<0.2) cathode material still has a problem during charge/discharge process, which is because a basic structural stability of the LiNiO2 typed cathode material is deteriorated.